It would be cool if NBGI released Sei Naru Yoru ni (I think that's the title?) as DLC. However, as we know from the past, NBGI is very cheap (Ex: Porting PSP DLC to L4U!). Since NBGI is cheap, they'll most likely release Kamisama no Birthday as the Christmas DLC since every idol so far (except for Hibiki and Takane) has sung the song. But yeah, Sei Naru Yoru ni is an awesome song.